Functionalized phenol-formaldehyde resin and method for treating metallic surfaces

ABSTRACT

The invention relates to a functionalized phenol-aldehyde resin which comprises a phenol-aldehyde condensation product, having a phenol component with no carboxyl group, and an aromatic hydroxycarboxylic acid and imidazole as constituents which is useful for corrosion-inhibiting treatment of bare or conversion-coated metal surfaces is provided. Methods of making the functionalized phenol-aldehyde resin, processes for coating metal surfaces using compositions according to the invention and products coated thereby are also included in the invention.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 USC Sections 365(c) and 120 of International Application No. PCT/EP2004/012324, filed 30 Oct. 2004 and published 7 Jul. 2005 as WO 2005/061570, which claims priority from German Application No. 103 58 309.2, filed 11 Dec. 2003, each of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to a new functionalized phenol-aldehyde resin and to the use of this or related resins for the anti-corrosion treatment of metal surfaces. The metal surfaces may be bare, i.e. non-pre-treated, metal surfaces or metal surfaces which already have a corrosion-inhibiting conversion layer. A particular feature of the invention in this regard is that no toxic chromium has to be used.

BACKGROUND OF THE INVENTION

Structural elements fitted together from metal sheets, such as vehicle bodies, housings of domestic appliances or metal furniture items, can be assembled from metal sheets which do not yet have a permanent corrosion-inhibiting coating. In a sequence of several process steps, a permanent corrosion-inhibiting coating consisting of a conversion layer and a paint layer can be produced after the metal parts have been assembled. A known example of this is the process sequence of phosphating and painting which is normally applied, for example, in car manufacture. The actual phosphating is only one step in a treatment chain which, besides cleaning and rinsing steps, generally comprises activation before phosphating, the actual phosphating and often a post-passivation after phosphating. These treatment steps are followed by several painting steps. Accordingly, the pre-treatment before painting requires several treatment steps which, in turn, necessitate a correspondingly extensive and hence expensive pre-treatment plant. In addition, waste containing heavy metals accumulates in the phosphating step and has to be expensively disposed of.

Besides phosphating, there are other known processes for producing a so-called conversion layer which protects the underlying metal against corrosion and which represents a primer for a subsequent paint layer. A “conversion layer” is understood to be a layer on a metal surface which is formed by “conversion treatment” under the effect of a “conversion solution” and which contains elements both from the metal surface and from the conversion solution. Typical examples are phosphate layers or chromating layers. Besides phosphating and chromating processes, there are other known processes for conversion treatment, for example using conversion solutions based on complex fluorides of boron, silicon, titanium or zirconium. These complex fluorides are mostly used together with organic polymers. Examples of such conversion treatments can be found in DE-A-101 31 723 and in the literature cited therein. However, none of these alternative processes has so far been able to replace phosphating as a pre-treatment before painting in car manufacture.

The deposition of corrosion-inhibiting layers on bare metal surfaces to increase protection against corrosion is the subject of numerous prior-art publications, of which some examples are mentioned in the following:

U.S. Pat. No. 5,129,967 discloses treatment baths for the no-rinse treatment (or “dried in place conversion coating”) of aluminium which contain

a) 10 to 16 g/l polyacrylic acid or homopolymers thereof,

b) 12 to 19 g/l hexafluorozirconic acid,

c) 0.17 to 0.3 g/l hydrofluoric acid and

d) up to 0.6 g/l hexafluorotitanic acid.

EP-B-8 942 discloses treatment solutions containing

a) 0.5 to 10 g/l polyacrylic acid or an ester thereof and

b) 0.2 to 8 g/l of at least one of the compounds H₂ZrF₆. H₂TiF₆ and H₂SiF₆, the pH value of the solution being below 3.5.

U.S. Pat. No. 4,992,116 describes treatment baths for the conversion treatment of aluminium with pH values of about 2.5 to 5 which contain at least three components:

-   a) phosphate ions in concentrations of 1.1×10⁻⁵ to 5.3×10⁻³ mol/l,     corresponding to 1 to 500 mg/l, -   b) 1.1×10⁻⁵ to 1.3×10⁻³ mol/l of fluoro acid of an element of the     group consisting of Zr, Ti, Hf and Si (corresponding to 1.6 to 380     mg/l, depending on the element) and -   c) 0.26 to 20 g/l of a polyphenol compound obtainable by reaction of     poly(vinylphenol) with aldehydes and organic amines in the form of a     Mannich reaction.     The amines used are open-chain amines, more particularly     polyhydroxyalkylamines.

WO 92/07973 teaches a chromium-free treatment process for aluminium which uses 0.01 to about 18% by weight of H₂ZrF₆ and 0.01 to about 10% by weight of a 3-(N—C₁₋₄-alkyl-N-2-hydroxyethylaminomethyl)-4-hydroxystyrene polymer as essential components in an acidic aqueous solution. Optional components are 0.05 to 10% by weight of dispersed SiO₂, 0.06 to 0.6% by weight of a solubilizer for the polymer and surfactant.

WO 97/31135 discloses a solution for the post-rinsing of conversion-treated metal surfaces which contains compounds, for example hexafluoro complexes, of Ti, Zr or Hf and a phenolic resin. The phenolic resin may contain differently substituted phenols. The molecular weight of the resin is in the range from 100 to 1,000.

U.S. Pat. No. 6,419,731 discloses a solution for the conversion treatment of aluminium which contains a zirconium compound, fluoride ions and a water-soluble resin. The water-soluble resin may be inter alia a phenolic resin.

U.S. Pat. No. 5,246,507 also relates to a composition for treating metal surfaces which contains metal compounds that may be selected, for example, from compounds of Ti, Zr and Hf and an organic polymer. The polymer may be, for example, a condensation product of formaldehyde with phenol and a phenolic carboxylic acid.

U.S. Pat. No. 5,846,917 describes phenolic imidazolines which can be obtained by a condensation reaction of hydrocarbyl polyaminophenols with carbonyl compounds. They are used primarily as antioxidants, but are also expected to show corrosion-inhibiting and passivating activity. These polymers differ structurally from phenolic resins in that they do not contain any alkylene-bridged phenol units.

Japanese patent application with the publication number 59-157110 (from Patent Abstracts of Japan) discloses phenolic resins which contain an imidazole ring. These resins are used as a component of heat-resistant adhesives for, for example, copper-containing laminates, circuit boards or the like.

Derwent Abstract AN (acquisition number) 1999-018521 contains a summary of Japanese document JP 10287859, according to which phenolic adhesives of the resol resin type additionally containing imidazole are produced. The adhesives are used for the production of plywood or veneer.

Phenol-aldehyde condensation products, more particularly phenol-formaldehyde condensation products, have long been known under the names of phenolic resins, phenolics, novolaks, resols, B-stage resins or C-stage resins. Information on their production and properties can be found, for example, in Römpps Chemie Lexikon under the keywords mentioned.

BRIEF SUMMARY OF THE INVENTION

The problem addressed by the present invention was to provide new, functionalized polymers of the phenolic resin type which can be used, more particularly in the form of an aqueous solution or emulsion, for the surface treatment of bare metal surfaces or metal surfaces already carrying a conversion layer. The treatment of the metal surfaces with the polymers improves corrosion prevention and/or the adhesion of a subsequently applied paint or an adhesive to the metal surface.

It is an object of the invention to provide a functionalized phenol-aldehyde resin containing a phenolic component with no carboxyl group, aromatic hydroxycarboxylic acid and imidazole as constituents. It is desirable that the functionalized phenol-aldehyde resin have a molar ratio of phenolic component to aromatic hydroxycarboxylic acid and the molar ratio of phenolic component to imidazole that are each in the range from 1:1 to 100:1. It is also desirable that the functionalized phenol-aldehyde resin have a molar ratio of aromatic hydroxycarboxylic acid to imidazole is in the range from 100:1 to 1:100. It is also independently desirable that the functionalized phenol-aldehyde resin has an average molecular weight of at least 500, more particularly at least 1,000, and at most 50,000, more particularly at most 10,000.

It is also an object of the invention to provide a functionalized phenol-aldehyde resin characterized in that at least 50% and preferably at least 90% of the phenolic component is phenol. It is also an object of the invention to provide a functionalized phenol-aldehyde resin wherein the aromatic hydroxycarboxylic acid is selected from hydroxybenzoic acids, more particularly from salicylic acid and p-hydroxybenzoic acid.

It is a further object of the invention to provide a functionalized phenol-aldehyde resin characterized in that at least 50% and preferably at least 90% of the resin consists of the phenolic component, aromatic hydroxycarboxylic acids and imidazole and the bridging alkylene groups.

Another aspect of the invention provides a process for the production of the functionalized phenol-aldehyde resin described herein, characterized in that aqueous aldehyde solution is added to an aqueous solution containing the phenolic component, aromatic hydroxycarboxylic acid and imidazole, followed by mixing for 10 minutes to 10 hours at a temperature between 40° C. and the boiling point.

Another aspect of the invention provides a process for the production of the functionalized phenol-aldehyde resin of the invention wherein:

a) in a first step, an aqueous solution of a phenol-aldehyde resin containing incorporated aromatic hydroxycarboxylic acids is prepared and

b) in a second step, imidazole and then aqueous aldehyde solution are added and the whole is mixed for 10 minutes to 10 hours at a temperature between 40° C. and the boiling point.

Desirably the aforementioned processes are characterized in that the concentration of the organic compounds in the aqueous reaction solution is selected so that an aqueous solution of the functionalized phenol-aldehyde resin of the invention with a solids content of 10 to 50% by weight is obtained.

In a different aspect of the invention, a process for the corrosion-inhibiting treatment of bare or conversion-coated metal surfaces is provided, characterized in that the metal surfaces are contacted with an aqueous treatment solution containing one or more phenol-aldehyde resins, characterized in that at least one phenol-aldehyde resin contains incorporated imidazole and is preferably the functionalized phenol-aldehyde resin as described herein. Desirably, the aqueous treatment solution contains at least 5, preferably at least 20 and up to 2,000, preferably up to 200 mg/l phenol-aldehyde resin which contains incorporated imidazole. It is also desirable that the aqueous treatment solution has a pH of at least 1.5, preferably at least 1.8 to at most 6, preferably to at most 4.5.

It is a further object of the invention to provide a one of the above-described processes wherein the aqueous treatment solution additionally contains fluoride ions and/or one or more compounds of elements of the fourth main or transition metal group of the periodic system, preferably compounds of Si, Ti and/or Zr.

It is also an object of the invention to provide a process for the production of a structural component containing painted metal parts, in which

a) sheets of metal with a coating based on organic polymers are cut and/or stamped and/or formed and the metal parts obtained are assembled to form the structural component, areas of the metal surface of the sheet that are not covered by the coating based on organic polymers being formed,

b) the assembled structural component is cleaned,

c) the cleaned, assembled structural component is contacted with a chromium-free aqueous treatment solution which produces a passivation layer that is not a zinc phosphate layer on those areas of the metal surface formed in step a) that are not covered by the coating based on organic polymers, the aqueous treatment solution containing one or more phenol-aldehyde resin(s), at least one phenol-aldehyde resin containing incorporated aromatic hydroxycarboxylic acids and/or incorporated imidazole and preferably being a functionalized phenol-formaldehyde resin as described herein,

d) if desired, the structural component treated in step c) is rinsed one or more times with water, although this is not essential, and

e) is coated with a paint layer.

In another aspect, the invention provides an aqueous treatment solution for treating bare or conversion-coated metal surfaces which contains one or more of the functionalized phenol-aldehyde resins according to the invention.

In yet another aspect of the invention a metal sheet, a metal part or an article containing metal parts, is provided characterized in that at least one surface of the metal sheet or the metal parts has been treated by the compositions and/or processes of the invention.

DETAILED DESCRIPTION OF THE INVENTION

A first aspect of the invention relates to a functionalized phenol-aldehyde resin which comprises a phenol-aldehyde condensation product, having a phenol component with no carboxyl group, and an aromatic hydroxycarboxylic acid and imidazole as constituents.

A “phenol-aldehyde resin” is understood in particular to be a phenol-formaldehyde resin. However, other aldehydes, such as furfural for example, may be used instead of, or in admixture with, the formaldehyde as aldehyde components. The phenolic component may be, above all, phenol itself. In a preferred embodiment, at least 50% and preferably at least 90% of the phenolic component is phenol. Instead of, or together with, phenol, other aromatic hydroxy compounds such as, for example, alkyl- or aryl-substituted phenols, for example cresols, polyhydric phenols, for example pyrocatechol, resorcinol or hydroquinone, trihydric phenols (pyrogallol, phloroglucinol, hydroxyhydroquinone) or anellated phenols, such as, for example, α- and β-naphthol, or alkyl-bridged diphenols, such as bisphenol A for example, may be used as the phenolic component.

The aromatic hydroxycarboxylic acid by definition has an aromatic ring system to which at least one hydroxy group and at least one carboxylic acid group are attached. The simplest examples of this are the position isomers of hydroxybenzoic acid, such as salicylic acid and m- or p-hydroxybenzoic acid. The aromatic ring system may carry further substituents such as, for example, alkyl groups, nitro groups, amino groups or even further hydroxy or carboxylic acid groups. The aromatic hydroxycarboxylic acid may even have a condensed aromatic ring system and may be, for example, one of the position isomers of hydroxynaphthoic acid. One example of an aromatic hydroxycarboxylic acid containing more than one carboxyl group is hydroxyphthalic acid. Whenever the term “aromatic hydroxycarboxylic acid” is mentioned in the present disclosure, it is always intended to mean that mixtures of different acids may also be present. The aromatic hydroxycarboxylic acid is preferably selected from hydroxybenzoic acids, more particularly from salicylic acid and p-hydroxybenzoic acid.

By “imidazole” is preferably meant the parent compound itself. However, the parent compound may carry substituents, more particularly at the C atoms. These substituents may represent another aromatic ring system, as in the case of benzimidazole for example.

With regard to the molar ratios of the individual constituents in the functionalized phenol-aldehyde resin, the molar ratio of phenolic component to aromatic hydroxycarboxylic acid and the molar ratio of phenolic component to imidazole are preferably selected independently of one another so that the proportion of phenolic component in the functionalized phenol-aldehyde resin is at least as large as, but preferably larger than, the proportion of aromatic hydroxycarboxylic acid or the proportion of imidazole. In a particularly preferred embodiment, the molar ratio of phenolic component to aromatic hydroxycarboxylic acid and the molar ratio of phenolic component to imidazole are selected independently of one another so that each is in the range from 1:1 to 100:1. The two molar ratios may of course be substantially the same or different. The molar ratios are preferably selected so that a molar ratio of aromatic hydroxycarboxylic acid to imidazole in the range from 100:1 to 1:100 and more particularly in the range from 10:1 to 1:10 is obtained. A molar ratio of 1:1 to 10:1 is especially preferred.

The composition of the functionalized phenol-aldehyde resin is preferably such that at least 50% and preferably at least 90% consists of the phenolic component, aromatic hydroxycarboxylic acid and imidazole and the bridging alkylene groups emanating from the aldehyde component (methylene groups where formaldehyde is used). The other constituents of the polymer may represent, for example, aromatic aminocarboxylic acids such as, in particular, aminobenzoic acids instead of the aromatic hydroxycarboxylic acid or other aromatic or aliphatic heterocycles instead of the imidazole. In another preferred functionalized phenol-aldehyde resin, at least 50%, preferably at least 90% and more particularly at least 100% of the phenolic component consists of the parent compound phenol, at least 50%, preferably at least 90% and more particularly 100% of the aromatic hydroxycarboxylic acid consists of a hydroxybenzoic acid (more particularly salicylic acid) and at least 50%, preferably at least 90% and more particularly 100% of the imidazole component consists of the parent compound, imidazole, itself. A functionalized phenol-aldehyde resin which consists entirely of phenol, hydroxybenzoic acid (more particularly salicylic acid), imidazole and the bridging alkylene groups (more particularly methylene groups) is particularly preferred. The average molecular weight of the functionalized phenol-aldehyde resin, which can be determined, for example, by gel permeation chromatography with polyethylene glycol as standard, is preferably at least 500, more particularly at least 1,000 and preferably at most 50,000 and more particularly at most 10,000.

The general process for the production of phenol-aldehyde resins by condensation of a phenol with an aldehyde in aqueous solution is generally known and is described in chemical textbooks and encyclopedias. The above-described functionalized phenol-aldehyde resins with their at least 3 aromatic components can be produced, for example, by adding aqueous aldehyde solution to an aqueous solution containing the phenolic component, aromatic hydroxycarboxylic acid and imidazole and mixing for 10 minutes to 10 hours at a temperature of 40° C. to the boiling point. For example, this solution may be boiled under reflux which, in itself, can produce an adequate mixing effect. Otherwise, mixing can be achieved by stirring or shaking. This production process represents another aspect of the present invention.

A more special production process according to the invention is characterized in that

-   a) in a first step, an aqueous solution of a functionalized     phenol-aldehyde resin containing incorporated aromatic     hydroxycarboxylic acids is prepared and -   b) in a second step, imidazole and then aqueous aldehyde solution     are added and the whole is mixed for 10 minutes to 10 hours at a     temperature between 40° C. and the boiling point.

In this process, too, the mixing of the solution both in the first step a) and in the second step b) can be achieved by refluxing, stirring or shaking.

In this production process, the concentration of the organic compounds in the aqueous reaction solution is preferably selected so that an aqueous solution of the functionalized phenol-aldehyde resin with a resin solids content of 10 to 50% by weight is obtained at the end of the reaction. For the intended use of the resin for treating metal surfaces, this solution need not be further purified. Rather, it can be directly marketed as such and used for the production of the treatment solution described in the following by dilution with water or for supplementing this treatment solution with active ingredient.

Another aspect of the present invention relates to the use of the functionalized phenol-aldehyde resin described above or of a mixture of two or more such resins for the corrosion-inhibiting treatment of bare metal surfaces or metal surfaces already carrying a conversion layer. The metal surfaces are preferably selected from surfaces of steel, galvanized or alloy-galvanized steel, aluminized steel, zinc, aluminium, magnesium or alloys of which at least 50 atom-% consists of zinc, aluminium or magnesium. The conversion layer on the metal surfaces may be, for example, an anodizing layer, a phosphating layer such as can be produced by a layer-forming or non-layer-forming phosphating process or a coating based on fluorocomplexes of, for example, B, Si, Ti, Zr, Hf, as described, for example, in the literature cited at the beginning.

The present invention also relates to a process for the corrosion-inhibiting treatment of bare metal surfaces or metal surfaces already provided with a conversion layer, in which the metal surfaces are contacted with an aqueous treatment solution containing one or more phenol-aldehyde resins, at least one phenol-aldehyde resin containing incorporated imidazole and preferably being a functionalized phenol-aldehyde resin as described herein.

Typical metal surfaces for this treatment are recited herein. The metal surfaces may be completely bare or completely coated with a conversion layer. However, they may also be metal surfaces of complex structural parts such as, for example, car bodies which consist partly of bare and partly of conversion-coated metal parts. A corrosion-inhibiting layer is then produced on the bare metal parts by the process according to the invention and the corrosion-inhibiting effect of the conversion-treated metal surfaces is improved. In addition, the metal surfaces may already partly carry an organic coating which is locally damaged, for example at cut edges, ground areas or spot welds, so that the metal surface is again bare at such places. Such conditions prevail, for example, when complex structural parts, such as car bodies or domestic appliances, are at least partly assembled from precoated sheets.

According to the invention, the minimum requirement in this regard is that at least one phenol-aldehyde resin is present which at least contains incorporated imidazole, but not necessarily the aromatic hydroxycarboxylic acid, too. The above-described preferred embodiments in regard to components and molar ratios apply accordingly to such phenol-aldehyde resins with incorporated imidazole, but without incorporated aromatic hydroxycarboxylic acid. However, the metal surfaces are preferably contacted with an aqueous treatment solution which contains at least one of the above-described functionalized phenol-aldehyde resins that comprises the three components: a phenol-aldehyde resin which contains a phenol component with no carboxyl group, aromatic hydroxycarboxylic acid and imidazole as constituents. Most preferably a functionalized phenol-aldehyde resin as described herein.

The aqueous treatment solution preferably contains at least 5 mg/l and, more particularly, at least 20 mg/l, but preferably at most 2,000 and, more particularly, at most 200 mg/l of phenol-aldehyde resin containing incorporated imidazole and preferably a functionalized phenol-aldehyde resin according to the invention. With lower contents, the corrosion-inhibiting effect diminishes increasingly. Even higher contents can adversely affect corrosion prevention or at least are uneconomical.

The aqueous treatment solution preferably has a pH of at least 1.5, more particularly of at least 1.8 to at most 6.0 and, more particularly, to at most 4.5. At lower pH values, the metal is increasingly too seriously attacked by the pickling effect. At higher pH values, layer formation increasingly deteriorates as, hence, does the corrosion-inhibiting effect.

The temperature of the treatment solution is preferably in the range from 20 to 60° C. and more particularly in the range from 25 to 40° C. The preferred contact time of the metal surface with the treatment solution is preferably in the range from 5 to 240 seconds and, more particularly, in the range from 30 to 200 seconds. The metal surface may be contacted with the treatment solution in the usual way, for example by immersion in the treatment solution, by spraying with the treatment solution or by combinations thereof or even by roller application of the treatment solution.

Where the process is used for the after treatment of metal surfaces which already have a conversion layer, the conversion layer on the one hand can have been produced immediately before the after treatment according to the invention and, accordingly, may still be wet. A rinse with water may be carried out between formation of the conversion layer and the after treatment according to the invention. However, it may also be omitted. On the other hand, however, a relatively long period of time may also elapse between formation of the conversion layer and the after treatment according to the invention. This is the case, for example, when structural parts, such as car bodies or domestic appliances for example, are assembled from prephosphated steel and then after treated by the treatment process according to the invention. A cleaning step may be inserted between the conversion treatment and the after treatment according to the invention.

In addition to the functionalized phenol-aldehyde resin, the aqueous treatment solution preferably contains one or more compounds of elements of the 4^(th) main or transition metal group of the periodic system, more particularly Si, Ti and/or Zr. A treatment solution containing in all at least 0.01 g/l, more particularly at least 0.025 g/l and up to 10 g/l, more particularly up to 1 g/l and especially up to 0.5 g/l Ti and/or Zr and/or Sn ions and at least such a quantity of fluoride that the atomic ratio of Ti to F and/or Zr to F and/or Si to F is in the range from 1:1 to 1:6 is preferably used.

The Ti, Zr and/or Si ions mentioned may be completely used in the form of hexafluoro complexes such as, for example, the hexafluoro acids or their salts soluble in water in the concentration range mentioned, such as the sodium salts for example. In this case, the atomic ratio is 1:6. However, complex compounds where less than six fluoride ions are attached to the central elements Ti, Zr or Si may also be used. These may be spontaneously formed in the treatment solution if both hexafluoro complexes of at least one of the central elements Ti, Zr or Si and at least one other compound of one of these central elements are added to the treatment solution. Suitable other compounds are, for example, nitrates, carbonates, hydroxides and/or oxides of the same central element or of another of the three central elements mentioned. For example, the treatment solution may contain hexafluorozirconate ions and (preferably colloidal) silica (SiO₂) or reaction products thereof. Unreacted silica may be suspended in the treatment solution.

A treatment solution such as this may also be obtained by using hydrofluoric acid or (optionally acidic) salts thereof together with compounds of Ti, Zr and/or Si which are capable of forming fluoro complexes therewith. Examples are the already mentioned nitrates, carbonates, hydroxides and/or oxides. A preferred embodiment is characterized in that, in all, such a quantity of Ti, Zr and/or Si as the central metal and such a quantity of fluoride are used that the atomic ratio of central metal to fluoride is 1:2 or lower, more particularly 1:3 or lower. The atomic ratio may even be lower than 1:6 if the treatment solution contains more fluoride, for example in the form of hydrofluoric acid or salts thereof, than is stoichiometrically necessary for forming the hexafluoro complexes of the central metals Ti, Zr and/or Si.

Depending on the substrate, the aqueous solution may additionally contain 0.001 to 2 g/l and preferably 0.005 to 0.5 g/l ions of one or more of the metals Mn, Ce, Li, V, W, Mo, Mg, Zn, Co and Ni. For environmental reasons, however, the use of Co and Ni ought to be avoided. These additional metal ions can further improve the corrosion-inhibiting effect and paint adhesion. In view of the pickling effect on the metal surfaces, the treatment solutions in practice will additionally contain metal ions that have been dissolved out from the metal surface. Apart from the zinc already mentioned, these metal ions may be, in particular, iron and aluminium. Their concentrations may also be in the range from 0.001 to 2 g/l and more particularly in the range from 0.005 to 0.5 g/l. In the treatment of aluminium surfaces in particular, it can be of particular advantage to add aluminium ions in the form of soluble aluminium compounds to the treatment solution from the outset in the concentration range mentioned. The same applies to the addition of zinc ions, for example as nitrate salt, in the treatment of galvanized substrates with the treatment solution.

In addition, the aqueous solution may additionally contain 0.001 to 1.5 g/l and preferably 0.1 to 1 g/l phosphoric acid, phosphorous acid, phosphonic acid and/or anions and/or esters thereof. Esters should be selected so that they are soluble or dispersible in water. These additions also improve the corrosion-inhibiting effect and paint adhesion. However, in line with the basic concept of the present invention, it is important to ensure that no combination of additions that would lead to the formation of a crystalline zinc-containing phosphate layer is selected, because this would lead to a conventional zinc phosphate layer which is known in the prior art and which only creates an adequate corrosion-inhibiting effect if the usual steps of activation and post-passivation are additionally carried out. However, the present invention seeks precisely to avoid this more complicated sequence of process steps. This is achieved, for example, if the treatment solution does not simultaneously contain zinc and/or manganese in concentrations of more than 0.3 g/l and phosphoric acid or phosphate ions in concentrations of more than 3 g/l.

However, it is of advantage if the aqueous solution additionally contains one or more of the components which are known in the phosphating field as so-called phosphating accelerators. This is particularly the case when the treatment solution is used for the treatment of bare metal surfaces. In phosphating, the principal function of such accelerators is to prevent the formation of bubbles of elemental hydrogen on the metal surface. This effect is also known as depolarization. The effect of depolarization in the process according to the invention, as in conventional phosphating, is that the conversion layer is formed more quickly and more uniformly. Accordingly, in a preferred embodiment, the aqueous solution contains one or more phosphating accelerators selected from

0.05 to 2 g/l m-nitrobenzenesulfonate ions,

0.1 to 10 g/l hydroxylamine in free or bound form,

0.05 to 2 g/l m-nitrobenzoate ions,

0.05 to 2 g/l p-nitrophenol,

1 to 70 mg/l hydrogen peroxide in free or bound form,

0.05 to 10 g/l organic N-oxides,

0.1 to 3 g/l nitroguanidine,

1 to 500 mg/l nitrite ions,

0.5 to 5 g/l chlorate ions.

The process according to the invention is generally incorporated in a treatment chain which usually begins with the cleaning of the parts to be treated. These parts may be bare metal parts which are coated by the treatment process according to the invention with a surface layer which improves corrosion prevention and the adhesion of a subsequently applied organic coating. The treatment with the treatment solution according to the invention may be the only treatment step which produces such a surface layer. However, the process according to the invention may also be used to improve corrosion prevention and paint adhesion on metal surfaces which are already coated with a conversion layer. This layer can have been applied by the manufacturer of the sheet material, so that a relatively long period of time can elapse between the first conversion treatment and the application of the treatment process according to the invention. However, the conversion layer may also be formed in an after treatment step immediately before application of the process according to the invention. Rinsing with water is generally carried out one or more times between the individual treatment steps and also after application of the process according to the invention. Deionized water is preferably used for the final rinse after application of the process according to the invention.

The metal surfaces treated by the process according to the invention are then generally coated with another layer based on organic polymers. This other layer may be, for example, a single- or multi-layer paint. For example, this paint may be a typical automotive paint of which the layer next to the metal is normally a cathodic dipping paint. However, the paint may also be applied as a powder which is regarded as adequate, for example, for domestic appliances, metal furniture and the like. The surface layer produced on the metal surfaces by the treatment process according to the invention may also serve as a primer for bonding. In this case, the treated metal surface is coated with an adhesive. Metal parts may be bonded to one another, to glass or to plastic parts or even to rubber. For example, the process may be used as a pre-treatment for rubber-to-metal bonding.

One particular potential application for the process according to the invention is described in the following:

In principle, it would be economically and ecologically advantageous to produce metal parts from material already precoated by the manufacturer of the metal sheets and simply to clean and paint those parts after assembly. Waste associated with the pre-treatment would thus accumulate in a single location, i.e. at the manufacturer of the metal sheets, and not more widely at the further processors of the metal sheets. Precoated metal sheets would thus be directly available on the market. On the one hand, they could be pre-phosphated, i.e. could carry a phosphate layer, but no other coating based on organic polymers. Metal sheets already provided with a corrosion-inhibiting layer by the manufacturer are also being processed to an increasing extent in the automotive and domestic appliance industries. Corresponding materials are known, for example, under the names of Granocoat®, Durasteel®, Bonazinc® and Durazinc®. They carry a thin organic coating over a conversion layer, for example a chromating or phosphating layer. The organic coating consists of polymer systems such as, for example, epoxy or polyurethane resins, polyamides and polyacrylates. Solid additives, such as silicas, zinc dust and carbon black, improve corrosion protection and, by virtue of their electrical conductivity, enable the metal parts coated with layers about 0.3 to about 10 μm and preferably up to about 5 μm thick to be electrically welded and electrolytically painted. The substrate materials are generally coated in a two-stage process, in which the inorganic conversion layer is first produced and the organic polymer film is then applied in a second treatment stage. More information on this process can be found in DE-A-100 22 075 and the literature cited therein.

Accordingly, metal sheets provided with a coating based on organic polymers by coil coating are already partly in use in the manufacture of vehicle bodies, domestic appliances and items of furniture. In car manufacture, corrosion prevention and paint adhesion have to meet the strictest requirements because vehicles are exposed to the most serious corrosive stresses. At present, no vehicle bodies are made exclusively from organically precoated metal sheets. Instead, this material is made up into the vehicle bodies together with non-precoated sheets. Accordingly, the assembled bodies at present still go through the usual pre-treatment process before painting, i.e. they are subjected to the expensive process of phosphating.

In principle, the phosphating process could be replaced by a less expensive pre-treatment process if the vehicle bodies were to be made exclusively from organically precoated metal substrate. However, this would necessitate finding a solution to the problem that, in the assembly of bodies from organically precoated metal sheets, areas are inevitably formed where the organic precoating is damaged or missing altogether. This is the case, for example, at cut edges, spot welds or ground areas.

In the interests of a better corrosion-inhibiting effect, organically precoated metal substrates where electrolytically galvanized or hot dip galvanized steel is used as the metal substrate are frequently used in vehicle manufacture. However, with organically coated metal substrates such as these, the areas mentioned where the organic layer is damaged are particularly difficult to handle because they differ from the usual metal surfaces in regard to their electrochemical potentials and their chemical reactivity. In damaged places such as these, parts both of the steel substrate (i.e. iron) and of the zinc coating generally remain bare. A high local area ratio of steel (iron) to zinc, for example a ratio of >9:1, can be present. This is the case in particular with cut edges which represent a cross-section through the coated steel substrate. At these border areas which combine zinc and iron, the corrosion ratios differ from the other ratios on the homogeneous surface. Depending on the local ratio of zinc to iron in the exposed metal areas, a different electrochemical potential is established between the potentials of zinc and iron. In addition, ground areas with special ratios and hence particular electrochemical potentials are formed in the treatment of bodies because an activated interface between steel (iron) and fine-particle reactive zinc is formed by the grinding process.

Now, another aspect of the present invention relates to a process for the production of a structural component containing painted metal parts, in which

-   a) sheets of metal (preferably galvanized steel) with a coating     based on organic polymers are cut and/or stamped and/or formed and     the metal parts obtained are assembled to form the structural     component, areas of the metal surface of the sheet that are not     covered by the coating based on organic polymers being formed, -   b) the assembled structural component is cleaned, -   c) the cleaned, assembled structural component is contacted with a     chromium-free aqueous treatment solution which produces a     passivation layer that is not a zinc phosphate layer on those areas     of the metal surface formed in step a) that are not covered by the     coating based on organic polymers, the aqueous treatment solution     containing one or more phenol-aldehyde resin(s), at least one     phenol-aldehyde resin containing incorporated aromatic     hydroxycarboxylic acids and/or incorporated imidazole and preferably     being the functionalized phenol-formaldehyde resin according to the     invention, -   d) if desired, the structural component treated in step c) is rinsed     one or more times with water, although this is not essential, and -   e) is coated with a paint layer.

In a preferred embodiment of the process for the production of a structural component, the treatment solution used in step c) comprises 20-200 mg/l of the functionalized phenol-aldehyde resin; and the treatment solution contains one or more compounds of elements of the fourth main or transition metal group of the periodic system, most preferably Si, Ti and/or Zr. Most preferably, the aqueous treatment solution additionally contains fluoride ions.

In this case, step c) is preferably the only treatment step after step a) which produces a passivation layer on those areas of the metal surface formed in step a) that are not covered by the coating based on organic polymers.

This special process may be used in particular when all the metal parts of the structural component subjected to steps b) to e) consist solely of the sheets of galvanized steel with a coating based on organic polymers.

Accordingly, all the metal parts of the structural component may consist of organically precoated metal, more particularly galvanized steel. In addition to these metal parts, however, the structural component may contain plastic components, as is the case, for example, in car manufacture. Accordingly, to produce a vehicle body for example, the metal parts of organically precoated material may be assembled together with plastic parts.

The term “galvanized steel” encompasses hot-dip-galvanized steels and electrolytically galvanized steels. It also encompasses alloy-galvanized steels where the coating may consist, for example, of a zinc/nickel alloy or a zinc/aluminium alloy. The steels may be tempered after galvanizing, so that an iron/zinc alloy is formed at the interface between steel and zinc.

The assembly of the sheets to form the structural component in step a) may be carried out by any of the usual known methods, for example by bonding, flanging, riveting, edging and/or welding, more particularly by electrowelding. Besides cutting and/or stamping in step a), joining by welding has the effect—attributable to the resulting damage to the coating based on organic polymers—that other areas not covered by the coating based on organic polymers are formed on the structural component. These other areas are also passivated in step c), as are bare metal areas formed by abrasion.

This embodiment of the process according to the invention is particularly suitable for the production of structural components using organically precoated sheets with a 1 to 10 μm thick coating based on organic polymers which contains electrically conductive particles in addition to the organic polymers. By virtue of these features of the organic coating, the structural components can be joined together by electrowelding. Examples of such coatings can be found in DE-A-197 48 764, DE-A-199 51 113, DE-A-100 22 075 and in the literature cited therein. As mentioned above, metal coils with such coatings are commercially available under various names.

Accordingly, the passivation layer produced in step c) is not intended to be a conventional zinc phosphate layer because a shortened and hence more economical treatment chain than zinc phosphating is intended to be used in accordance with the present invention. A zinc phosphate layer is not formed if the treatment solution does not simultaneously contain at least 0.3 g/l zinc ions and at least 3 g/l phosphate ions (as phosphoric acid or any pyrolysis stage thereof).

In step c), the assembled structural component can be contacted with the acid aqueous treatment solution in various ways, for example by immersion in the treatment solution or by spraying with the treatment solution. This step may be followed by rinsing with water although this is not imperative. In other words, the process may be used as a rinse process or as a no-rinse process.

The treatment in step c) is preferably not a post-passivation of a conversion layer formed primarily in a preceding step, but is the only treatment step after assembly of the parts which produces a passivation layer on the bare metal areas.

This sequence of process steps may be used in particular in the manufacture of vehicle bodies, domestic appliances, items of furniture or parts thereof.

Another aspect of the present invention relates to an aqueous treatment solution for treating bare or conversion-coated metal surfaces which contains at least one functionalized phenol-aldehyde resin according to the invention, as characterized in detail herein, or a mixture of two or more such resins. In a preferred embodiment, the treatment solution used in step c) comprises 20-200 mg/l of the functionalized phenol-aldehyde resin; and the treatment solution contains one or more compounds of elements of the fourth main or transition metal group of the periodic system, most preferably Si, Ti and/or Zr. Most preferably, the aqueous treatment solution additionally contains fluoride ions.

Finally, the present invention also relates to a metal sheet, a metal part or an article containing metal parts, characterized in that at least one surface of the metal sheet or the metal parts has been treated by the process according to the invention. As described above, the metal parts may have a coating based on organic polymers, i.e. for example a paint or adhesive, on the treated surfaces.

EXAMPLES

Abbreviations:

s=seconds; mins=minutes; h=hours; d=days

DE water=deionized water; SS=salt spray test;

CRS=cold-rolled steel

Ridoline® and Ridosol® are alkaline cleaners from Henkel KGAA.

Syntheses:

The quantitative ratios of the resin components are molar ratios.

1. Production of Phenol-Salicylic Acid-Formaldehyde Resins

a. Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 3:1)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution of 20 g phenol, 9.78 g salicylic acid and 37.8 g 30% sodium hydroxide is prepared at 80° C. After the phenol has completely dissolved, 23.3 g of a 36.5% formaldehyde solution are added over a period of 30 minutes from a dropping funnel. The solution is then heated to 95° C. and stirred for 6 h.

Solids content: 42.4%

b. Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 1:1)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution of 10 g phenol, 14.68 g salicylic acid and 28.3 g 30% sodium hydroxide is prepared at 80° C. After the phenol has completely dissolved, 17.5 g of a 36.5% formaldehyde solution are added over a period of 30 minutes from a dropping funnel. The solution is then heated to 95° C. and stirred for 6 h.

Solids content: 42.2%

c. Imidazole-Modified Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 3:1, 0.5 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 44.5 g phenol-salicylic acid-formaldehyde resin (phenol/salicylic acid 3:1, see 1.a.) and 3.62 g imidazole is prepared at 80° C. After the educts have completely dissolved, 3.8 g of a 36.5% formaldehyde solution in 10 g water are added over a period of 15 minutes from a dropping funnel. The solution is then heated to 95° C. and stirred for 6 h.

Solids content: 40.6%

d. Imidazole-Modified Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 1:1, 0.5 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 39.5 g phenol-salicylic acid-formaldehyde resin (phenol/salicylic acid 1:1, see 1.b.) and 1.81 g imidazole is prepared at 80° C. after the educts have completely dissolved, 1.89 g of a 36.5% formaldehyde solution in 10 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h.

Solids content: 36.5%

e. Imidazole-Modified Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 1:1, 0.33 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 39.5 g phenol-salicylic acid-formaldehyde resin (phenol/salicylic acid 1:1, see 1.b.) and 1.21 g imidazole is prepared at 80° C. after the educts have completely dissolved, 1.26 g of a 36.5% formaldehyde solution in 10 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h.

Solids content: 35.0%

f. Imidazole-Modified Phenol-Salicylic Acid-Formaldehyde Resin (Phenol/Salicylic Acid 1:1, 0.25 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 39.5 g phenol-salicylic acid-formaldehyde resin (phenol/salicylic acid 1:1, see 1.b.) and 0.9 g imidazole is prepared at 80° C. after the educts have completely dissolved, 0.95 g of a 36.5% formaldehyde solution in 10 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h.

Solids content: 35.3%

2. Production of Phenol-Formaldehyde Resins

a. Phenol-Formaldehyde Resin

In a three-necked flask equipped with a reflux condenser and stirrer, a solution of 20 g phenol and 28.3 g 30% sodium hydroxide is prepared at 80° C. after the phenol has completely dissolved, 15.1 g of a 36.5% formaldehyde solution are added over a period of 30 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h.

Solids content: 43.3%

b. Imidazole-Modified Phenol-Formaldehyde Resin (0.25 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 10 g phenol-formaldehyde resin (see 2.a.) and 0.46 g imidazole is prepared at 80° C. after the educts have completely dissolved, 0.48 g of a 36.5% formaldehyde solution in 25 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h. Solids content: 10.3%

c. Imidazole-Modified Phenol-Formaldehyde Resin (0.5 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 10.2 g phenol-formaldehyde resin (see 2.a.) and 1.25 g imidazole is prepared at 80° C. after the educts have completely dissolved, 1.31 g of a 36.5% formaldehyde solution in 25 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h.

Solids content: 13.4%

d. Imidazole-Modified Phenol-Formaldehyde Resin (1 Imidazole)

In a three-necked flask equipped with a reflux condenser and stirrer, a solution consisting of 10 g phenol-formaldehyde resin (see 2.a.) and 1.84 g imidazole is prepared at 80° C. after the educts have completely dissolved, 1.93 g of a 36.5% formaldehyde solution in 25 g water are added over a period of 15 minutes from a dropping funnel. the solution is then heated to 95° C. and stirred for 6 h. Solids content: 13.0%

3. Use of the New Polymers for the Conversion Treatment

Example 1 Conversion Process with Modified Resins

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, polymer     -   (Table 1, 37 mg/l solids content); pH 4.0 -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);     -   ca. 60-80 μm         Corrosion Test:         Neutral Salt Spray Test SS DIN 50021, 21d

Note on evaluation of the results: the creepage values at the cut were determined from one side of the cut (in accordance with the Standard). in some test series, Comparative Example 1b with Sokalan® HP 56 was used as the reference system. this corresponded to a relative corrosion protection of 1. These “reference sheets” were included in each corrosion test series in order to obtain comparability between various test series. Systems with a better corrosion protection than the reference system have a relative corrosion protection (“rel. corrosion protection”) of <1, while those with a poorer corrosion protection have a relative corrosion protection of >1. in other test series, the result is directly expressed as the degree of paint creepage. this is apparent from the column headings of the Tables. TABLE 1 Example 1, Comparative Example 1 Polymer skeleton (molar ratio phenol:salicylic Amine (%, based Rel. corrosion acid = 1:1) on phenol) protection Phenol-salicylic 0 (Comparative 0.77 acid-formaldehyde Example 1) Phenol-salicylic 0.25 imidazole 0.70 acid-formaldehyde (invention) Phenolic-salicylic 0.33 imidazole 0.74 acid-formaldehyde (invention) Phenol-salicylic 0.5 imidazole 0.74 acid-formaldehyde (invention)

Example 2 and Comparative Example 1a

Pre-Treatment with Non-Carboxylic Acid-Modified Phenol-Formaldehyde Resins

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, polymer     -   (Table 2, 37 mg/l solids content); pH 4.0 -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);     -   ca. 60-80 μm         Corrosion Test:

Neutral Salt Spray Test SS DIN 50021, 21d TABLE 2 Example 2 (phenol-formaldehyde-imidazole resins), Comparative Example 1a (phenol-formaldehyde resins without imidazole) Molar ratio Rel. corrosion Polymer skeleton Amine imidazole:phenol protection Phenol-formaldehyde — 0 Not applicable (Comparative 1a) due to poor stability of the polymer in the solution used Phenol-formaldehyde Imidazole 0.25 1.2 Phenol-formaldehyde Imidazole 0.5 1.2 Phenol-formaldehyde Imidazole 0.5 1.5 Phenol-formaldehyde Imidazole 1 1.08

Comparative Example 1b

Pre-Treatment with Homo- and Copolymers of Polyvinyl Pyrrolidone (Using Sokalan® HP 56³): Reference System for the “Relative Corrosion Protection”)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, polymer     -   (Table 3, 37 mg/l solids content); pH 4.0 -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);     -   ca. 60-80 μm         Corrosion Test:

Neutral Salt Spray Test SS DIN 50021, 21d TABLE 3 Comparative Example 1b Rel. Vinyl Vinyl Vinyl corrosion Polymer pyrrolidone caprolactam imidazole protection Luvitec ® K¹⁾ 1 0 0 >>1 Luvitec ® 1 1 0 >>1 VPC 55K65W²⁾ Sokalan ® HP 56³⁾ 1 0 1 1 ¹⁾Polyvinyl pyrrolidone. Manufacturer: BASF; CAS No. 9003-39-8 ²⁾N-vinylcaprolactam-1-vinyl-2-pyrrolidone copolymer. Manufacturer: BASF; CAS No. 51987-20-3 ³⁾Vinyl imidazole-vinyl pyrrolidone copolymer. Manufacturer: BASF; CAS No. 117197-37-2 was used here as the standard system

Comparative Example 1c Iron Phosphating

Pre-treatment was carried out by one of applicants' standard methods. the paint used was Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP); ca. 60-80 μm (as above). Relative corrosion protection >1.

Comparative Example 1d Di-Cation Zinc Phosphating

Pre-treatment was carried out by one of applicants' standard methods. the paint used was Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP); ca. 60-80 μm (as above). Relative corrosion <1.

Example 3

Conversion Process with Modified Resins at Different pH Values

Polymer: Phenol-Salicyclic Acid-Formaldehyde Grafted with 0.5 Part Imidazole, Based on Phenol (Ration of Phenol to Salicylic Acid 1:1)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, Table 4 (37 mg/l polymer         solids content) -   5. Rinse: DE water -   6Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);

ca. 60-80 μm TABLE 4 Example 3 Corrosion protection Test pH (DIN 50021 SS, 21 d) U/2 in mm 1 1.8 1.2 2 2.0 2.0 3 2.2 2.2 4 2.4 2.0 5 4.0 2.7

Example 4

Conversion process with resin: phenol-formaldehyde grafted with 0.25 part imidazole, based on phenol; different pH values.

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, Table 5 (37 mg/l polymer         solids content) [phenol-formaldehyde resin×0.25 imidazole] -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);

ca. 60-80 μm TABLE 5 Example 4 Corrosion protection Test pH (DIN 50021 SS, 21 d) U/2 in mm 1 1.8 2.0 2 4.0 3.2

Example 5

Conversion process based on a combination of H₂ZrF₆ and SiO₂ at different pH values; painting: powder coating or cationic electrocoat paint (CEP)

Polymer: phenol-salicylic acid-formaldehyde grafted with 0.5 part imidazole, based on phenol (ratio of phenol to salicylic acid 1:1)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr; addition of SiO2     -   (Table 6, 37 mg/l polymer solids content) -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);

ca. 60-80 μm; or CEP from BASF coatings, Cathoguard 310 (lead-free, ca. 20 μm) TABLE 6 Example 5 Powder coating CEP (DIN 50021 (DIN 50021 Addition of SS, 21 d) SS, 21 d) Test pH SiO₂ mg/l U/2 in mm U/2 in mm Ex. 5a 2.8 No (invention, 2.0 1.6 but embodiment without SiO₂) Ex. 5b 2.8 250 mg/l 1.2 — Ex. 5c 4.0 250 mg/l 1.7 1.3 Comp. Ex. 3 4.0 No 2.5 1.8 (see below)

Comparative Example 3

Conversion Process with Polyvinyl Pyrrolidone/Polyvinyl Imidazole Copolymer (Sokalan® HP 56)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with ZrF₆ ²⁻ corresponding to     150 mg/l Zr; (37 mg/l polymer solids content) -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);     -   ca. 60-80 μm; or CEP from BASF Coatings, Cathoguard 310         (lead-free, ca. 20 μm)

Example 6

Conversion Process Based on H₂ZrF₆ with Different Quantities of Polymer

Polymer: Phenol-Salicylic Acid-Formaldehyde Grafted with 0.5 Part Imidazole, Based on Phenol (Ratio of Phenol to Salicylic Acid 1:1)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with one of the following     baths:     -   ZrF₆ ²⁻ corresponding to 150 mg/l Zr, addition of polymer (Table         7); pH 1.8 -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP);

ca. 60-80 μm TABLE 7 Example 6 and Comparative Examples 4a and 4b Addition of Powder coating polymer mg/l (DIN 50021 SS, Test solids 21 d) U/2 in mm Comp. Ex. 4a 0 2.6 Ex. 6a 40 1.3 Ex. 6b 100 1.1 Ex. 6c 500 1.9 Ex. 6d 1000 2.1 Comp. Ex. 4b 37 mg/l 3.0 (see below) Sokalan ® HP56

Comparative Example 4

Conversion Process with Polyvinyl Pyrrolidone/Polyvinyl Imidazole Copolymer (Sokalan® HP 56)

Substrate: CRS

Process Step Sequence (Dip Application)

-   1. Cleaning: Ridoline 1570, 2%; Ridosol 1237, 0.3%; 5 mins.; 55° C. -   2. Rinse: water -   3. Rinse: DE water -   4. Conversion treatment: 180 s; 30° C. with ZrF₆ ²⁻ corresponding to     150 mg/l Zr, pH 4.0 -   5. Rinse: DE water -   6. Drying: compressed air -   7. Painting: Polyester PES 5807/RAL 5009 GL (TIGC-free, from IGP) 

1. a functionalized phenol-aldehyde resin comprising: a) a phenolic component having no carboxyl functional group; b) an aromatic hydroxycarboxylic acid; and c) an imidazole.
 2. the functionalized phenol-aldehyde resin as claimed in claim 1, wherein the molar ratio of phenolic component to aromatic hydroxycarboxylic acid and the molar ratio of phenolic component to imidazole are each in the range from 1:1 to 100:1.
 3. the functionalized phenol-aldehyde resin as claimed in claim 1, wherein the molar ratio of aromatic hydroxycarboxylic acid to imidazole is in the range from 100:1 to 1:100.
 4. the functionalized phenol-aldehyde resin as claimed in claim 1, wherein at least 50% of the phenolic component is phenol.
 5. the functionalized phenol-aldehyde resin as claimed in claim 1, wherein the aromatic hydroxycarboxylic acid is selected from hydroxybenzoic acids.
 6. the functionalized phenol-aldehyde resin as claimed in claim 1, wherein at least 50% of the resin consists of the phenolic component, aromatic hydroxycarboxylic acid, imidazole and bridging alkylene groups.
 7. the functionalized phenol-aldehyde resin as claimed in claim 1 having an average molecular weight of at least
 500. 8. a process for the production of the functionalized phenol-aldehyde resin claimed in claim 1, comprising adding an aqueous aldehyde solution to an aqueous solution containing the phenolic component, aromatic hydroxycarboxylic acid and imidazole, thereby forming an aqueous reaction solution followed by mixing the aqueous reaction solution for 10 minutes to 10 hours at a temperature between 40° C. and the boiling point of the aqueous reaction solution.
 9. a process for the production of the functionalized phenol-aldehyde resin claimed in claim 1, comprising: a) preparing an aqueous solution of a phenol-aldehyde resin containing incorporated aromatic hydroxycarboxylic acids; and b) adding imidazole and then aqueous aldehyde solution to the aqueous solution of a) and mixing for 10 minutes to 10 hours at a temperature between 40° C. and the boiling point of the mixture.
 10. the process as claimed in claim 8, further comprising selecting the concentration of the organic compounds in the aqueous reaction solution such that the functionalized phenol-aldehyde resin produced has a solids content of 10 to 50% by weight.
 11. a process for the corrosion-inhibiting treatment of bare or conversion-coated metal surfaces, comprising contacting bare or conversion-coated metal surfaces with an aqueous treatment solution containing one or more phenol-aldehyde resins, wherein at least one phenol-aldehyde resin comprises the functionalized phenol-aldehyde resin claimed in claim
 1. 12. the process as claimed in claim 11, wherein the one or more phenol-aldehyde resins comprises 5-2,000 mg/l phenol-aldehyde resin which contains incorporated imidazole.
 13. the process as claimed in claim 12, wherein the one or more phenol-aldehyde resins comprises 20-200 mg/l of said functionalized phenol-aldehyde resin.
 14. the process as claimed in claim 11, wherein the aqueous treatment solution additionally contains one or more compounds of elements of the fourth main or transition metal group of the periodic system.
 15. the process as claimed in claim 14, wherein said elements of the fourth main or transition metal group of the periodic system comprise Si, Ti and/or Zr.
 16. the process as claimed in claim 15, wherein the aqueous treatment solution additionally contains fluoride ions.
 17. a process for the production of a structural component containing painted metal parts, in which a) sheets of metal with a coating based on organic polymers are cut and/or stamped and/or formed into metal parts, areas of metal surface of the sheet that are not covered by the coating based on organic polymers being formed thereby, b) the metal parts obtained are assembled to form the structural component, and the assembled structural component is cleaned, c) the cleaned, assembled structural component is contacted with a chromium-free aqueous treatment solution which produces a passivation layer that is not a zinc phosphate layer on said areas of metal surface formed in step a) that are not covered by the coating based on organic polymers, the aqueous treatment solution containing one or more phenol-aldehyde resin(s), at least one phenol-aldehyde resin comprising the phenol-formaldehyde resin claimed in claim 1, d) optionally, the structural component treated in step c) is rinsed one or more times with water, and e) is coated with a paint layer.
 18. an aqueous treatment solution for treating bare or conversion-coated metal surfaces which contains one or more of the functionalized phenol-aldehyde resins claimed in claim
 1. 19. a metal sheet, a metal part or an article containing metal parts, wherein at least one surface of the metal sheet or the metal parts has been treated by the process claimed in claim
 11. 